1. Field of the Invention
The present invention relates to a method for the preparation of a fluoride containing conductive coated substrate, e.g. glass.
The present invention further relates to the decomposition, e.g., by combustion, of fluorocarbon in the presence of an oxygen source to prepare hydrogen fluoride as a dopant source in the preparation of a conductive coated substrate, e.g. glass.
More particularly, the present invention relates to a method of forming a fluoride containing conductive metal oxide, e.g., tin oxide, coated substrate, e.g. glass.
2. Description of the Related Art
The preparation of transparent conducting coatings, e.g. TCOs (transparent conductive oxides) from sources such as tin, indium and zinc compounds in the presence of an oxygen source is well known. Methods of preparing conductive oxide coatings require the introduction of a coating source gas, e.g., tin, indium or zinc compounds, such as tin tetrachloride, tetramethyltin, trimethylindium or diethylzinc, o a coating furnace. The source gas is deposited in the presence of an oxygen source, such as O.sub.2, water, alcohol and the like, which reacts with the source compound, thereby forming a coating of, for example, tin oxide, indium oxide or zinc oxide.
Often these coatings are doped with a fluoride component, e.g., HF, to improve their performance, e.g., conductivity. A multitude of fluoride sources have been used in the prior art methods of forming doped conductive coated glass.
Routinely, gas blends of hydrogen fluoride in pressurized cylinders, typically 0.5-1.5% hydrogen fluoride in nitrogen, are used, but have the drawbacks of being expensive, corrosive and toxic. Systems based on liquid hydrogen fluoride have been found to be more cost effective, but the problems of corrosion, particularly with flow controllers, and safety associated with storing and handling of a toxic compound (hydrogen fluoride has a boiling point of 19.5.degree. C.) continue to plague the industry.
The prior art generally recognizes the ability to decompose chlorofluorocarbons, chlorohydrocarbons and fluorocarbons in the presence of oxygen by a combustion reaction and to thereby obtain as one of the by-products, a small amount of hydrogen fluoride. The prior art, however, has never recognized the concept of feeding such a combustion product stream into a furnace in the preparation of conductive coated substrates, e.g., glass products.
Some in the field have attempted the direct injection of fluorocarbons into a tin oxide coating furnace but under the temperature and flow conditions that are utilized in such furnaces, only a small amount, less than 1%, of most fluorocarbons are decomposed. The direct injection of fluorocarbons into the tin oxide coating furnace requires excessively large flows of reactant gas to get the necessary levels of dopant to result in sufficient fluoride doping and high conductivity. Because so little of the fluorocarbons are decomposed and because the release into the air of large amounts of fluorocarbons is undesirable in light of recent environmental concerns about the effect of chlorofluorocarbons in the atmosphere, this method has heretofore been unacceptable.
The present inventors have devised a simple and effective method for fluoride doping of transparent conductive coatings.
The inventors have further devised a cost effective and safe method of preparing high yields of fluoride source gas from fluorocarbons.